Intermediate System to Intermediate System (IS-IS) is an Interior Gateway Protocol (IGP) defined in International Organization for Standardization (ISO)/International Engineering Consortium (IEC) 10589 and in Internet Standard RFC 1142 published by the Internet Engineering Task Force (IETF). IS-IS may be used for link state routing in packet switching networks. For link state routing, the nodes construct a map of the connectivity to the network, e.g., in the form of a graph, showing which nodes are connected to which other nodes. Each node may then independently calculate best paths from it to every possible destination in the network. The collection of best paths forms the node's routing table.
In IS-IS, different IS-IS interfaces on the same device are identified by using different IS-IS interface IDs. The types of interfaces supported by IS-IS mainly include Broadcast Network, Point-to-Point (P2P) network, and Non-Broadcast Multi-Access (NBMA) network. NBMA links are configured as sub-interfaces to support IS-IS, and the type of the sub-interface is P2P or Broadcast Network.